ECE and OE [1]}, [2]}, [3]} are proper scoring rules (not strict) and are adapted from reliability diagrams for judging the calibration of the models. They are not strict as the optimum value of 0 can be achieved with more than one set of predictions. These metrics also suffer from high sensitivity to the bin hyper-parameter [4]}. Finding a good calibration metric is an active area of research [5]}, [4]}
